Do trains use AC or DC motors?

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thenorthern

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This may seem a strange question but do EMUs use DC or AC motors or universal ones and what about the dual voltage trains what do they use?

Also what type of motor do the DEMUs use?
 
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broadgage

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Yes, or in more detail, DC third rail EMUs USED to use DC motors, and some are still in use. Speed control was achieved by switching the motors in series or parallel and by inserting or cutting out series resistances. Simple and effective but wasteful of power, and a lot of contactors to maintain, and brushes on the motors to replace.

Modern practice is to use AC motors fed with three phase AC from an inverter. The voltage and frequency are varied electronically with but little loss, and few moving parts and no brushes.
Early designs proved unreliable, but recent types are much better.

AC locos and EMUs followed broadly similar practice after transforming the AC to a lower voltage and then rectifying it to DC.
 
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PowerLee

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AC motors are a lot less lifetime cost & hassle to look after plus you can get more power from a smaller lighter motor then the equivalent DC motor.

Ive been working with both DC & AC motored industrial equipment ( not trains ) for 10+ years.

Normal DC traction motors require carbon brushes, occasional commutator skim so the brushes work correctly, carbon dust removal from inside the motor housing to stop short circuits & the armature bearings dont seem to last as long.

AC traction motors apart from blowing out the dust from the motor housing & replacement bearings are a lot easier to look after.
 

greaterwest

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The SWT class 455 fleet (built circa 1982) is being fitted with AC traction motors currently.
 

yorkie

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The SWT class 455 fleet (built circa 1982) is being fitted with AC traction motors currently.
Does this mean they'll lose their classic sound?
--- old post above --- --- new post below ---
Technically speaking, the 1983 Tube Stock had AC Electric Motors controlling the camshaft for a DC Motor ;)
Sounds complex... was this part of the reason for their reported unreliability?
--- old post above --- --- new post below ---
Also what type of motor do the DEMUs use?
As above, newer ones such as Voyagers are AC, while older ones such as Thumpers are DC.
 

edwin_m

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Some countries used universal motors fed with AC, in the days before the invention of rectifiers that were suitable for use on board trains. This meant the train could carry a transformer and be fed at a higher supply voltage, reducing the number of substations and the transmission losses. However the universal motors available at the time wouldn't run at 50Hz and this is the reason Germany, Austria and Switzerland adopted and still use 16.67Hz for their main line railway electrification.

Most AC-motored trains use asynchronous induction motors, but a few (particularly French ones such as TGV Atlantique) use synchronous motors where the frequency supplied to the motors has to match the rotation speed exactly. I read somewhere that the synchronous motor may make a comeback, but using permanent magnets, as new materials allow these to generate much greater magnetic fields than before.
 

thenorthern

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Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.
 

AlexNL

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I expect the power supply in the southern region to come from the national grid, so yes... generated as AC, rectified to DC and then fed into the traction motors. Whether or not the power is then transformed back to AC depends on the particular type of train.
 

rebmcr

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Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.

Not really, and it's not unique either. A modern overhead-pickup EMU takes generated standard AC from the pantograph, and rectifies it to DC so the thyristor can create a varied range of traction AC from it.
 

DerekC

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Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.

There is certainly a large waste of energy in the 3rd rail system but it's mostly resistive losses because of high current in the steel conductor rail (and running rails) rather than in transformers and rectifiers.
 

375610

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I read somewhere that the synchronous motor may make a comeback, but using permanent magnets, as new materials allow these to generate much greater magnetic fields than before.

My washing machine has a permanent magnet AC motor, apparently.
 

Nym

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Sounds complex... was this part of the reason for their reported unreliability?
--- old post above --- --- new post below ---

In the same way that AEI equipment was when first introduced, or anything else. When you only have around 100 of something very complex, it's always difficult to maintain.

Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.

It is, but it isn't. Is the same as on AC OHLE.

There is certainly a large waste of energy in the 3rd rail system but it's mostly resistive losses because of high current in the steel conductor rail (and running rails) rather than in transformers and rectifiers.

Or aluminium where fitted.
 

jopsuk

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Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.

On a grander scale, the interconnections across the sea that link the GB mainland power grid to the Ireland and Europe grids are High Voltage Direct Current. For mid-range transmission AC is more efficient, but under sea and at very high voltage there's losses unique to AC and DC becomes the more efficient system. Modern power converters minimise the losses
 

SpacePhoenix

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For an EMU running of the 3rd rail, is there much difference in the weight of the traction package between an AC package and a DC package?
 

Nym

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For an EMU running of the 3rd rail, is there much difference in the weight of the traction package between an AC package and a DC package?

There isn't a fair comparison, you'd need to compare an IGBT DC Chopper to an IGBT Invertor package, the former isn't really made.

DC motors tend to be heavier than Induction Machines.
AC invertors do need more filters to work in a 'lab enviroment' but in the real world, DC motors need big filters too.
 

Taunton

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For an EMU running of the 3rd rail, is there much difference in the weight of the traction package between an AC package and a DC package?
Yes, the difference can be substantial. The AC emu is effectively carrying round its own substation with it. There are a lot of downsides to this which are generally glossed over by the anti-third rail faction, but a 12-car AC emu is carrying round three substations under the motor unit frames. The difficulties referred to earlier with the 1960s AEI EMUs in Glasgow and East London were a combination of poor design and the need to get round the patent on the best way to do it which English Electric, who equipped the other half of the AC EMUs involved, had registered. In more recent designs these problems went away but were replaced by innumerable difficulties with the solid state AC control gear causing frequency interference with signalling.

You may notice that tramways, universally, including new systems, which run in the street are not allowed to have AC, and all use DC.
 
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notadriver

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There isn't a fair comparison, you'd need to compare an IGBT DC Chopper to an IGBT Invertor package, the former isn't really made.



DC motors tend to be heavier than Induction Machines.

AC invertors do need more filters to work in a 'lab enviroment' but in the real world, DC motors need big filters too.



An example of this is the class 378/1s designed for DC only compared with the 378/2s which are dual voltage with a transformer. The 378/1s were lighter and had a slight performance edge over the 378/2s. Another example is the 375/6s which have AC equipment vs other their sub classes. Again there is a slight difference.

Does anyone know what the advantage is of the new permanent magnet motors and in particular why they are able to produce more power under lower supply voltages than older types ?
 

snowball

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You may notice that tramways, universally, including new systems, which run in the street are not allowed to have AC, and all use DC.
Are trams really not allowed to use AC and why?
I understand, of course, that the supply to trams is not allowed to use a voltage higher than 750 V, and that it is normally DC, but that doesn't explain why an AC supply would be banned.
 

hwl

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Are trams really not allowed to use AC and why?
I understand, of course, that the supply to trams is not allowed to use a voltage higher than 750 V, and that it is normally DC, but that doesn't explain why an AC supply would be banned.

Less power at the same voltage so higher currents and larger cables, plenty of practical reasons not to use AC for trams!
 

gimmea50anyday

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DC traction for a number of years used the EE507 motor built by English Electric and continued to be manufactured by GEC (as GEC took over EE in the 1970's and is now part of alsthom) until fairly recently. The most recently built traction to employ this motor is the class 456. Derivatives of this motor were also found in EE built locomotives' (EE538 in 37, 50 and 55 and Irish class 80) and in other EMU's (EE546 in 73 and REP/442) no doubt the DC motors built by GEC (such as the 282az) for AC mk3 based stock being updated from the EE507

The first traction in this country to employ AC motors is the networkers. A 210 was converted into a test EMU utilising a pan car from a 313 to trial the traction equipment which proved to be a success. Sadly only one car remains of the test EMU in preservation as two other cars were converted to replace damaged 455 vehicles. Now AC is used as standard as the power to weight is higher meaning more and smaller motors can be fitted thereby improving tractive effort and acceleration in the process. Less maintenance is required as there are less moving parts plus AC doesn't suffer from field generation at high speed whereas DC motors can act as power generators reducing their power
 

AM9

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... The 378/1s were lighter and had a slight performance edge over the 378/2s. Another example is the 375/6s which have AC equipment vs other their sub classes. Again there is a slight difference...

In theory that may be true but when their performance is actually used, the maximum power drawn by those running on DC is limited by software to as little as 50% of their full performance, - then there is the tendency for the DC supply to fall well below its nominal voltage when under load.
Under ac, the same designs can use their full power almost without restriction. There are a few ac branch lines and ends of lines that have been electrified on the cheap, (like the Ely-Kings Lynn stretch) but the DC lines also have jokes like Poole to Weymouth to contend with. That's over 30 miles of pseudo-mainline with branchline quality DC electrification.
 

Elecman

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Less power at the same voltage so higher currents and larger cables, plenty of practical reasons not to use AC for trams!

Assume your referring to power factor there, that of course can be countered by use of power factor correction capacitors so getting the power factor upto a very acceptable level thus not "wasting" reactive power.

Don't forget that DC requires rectifiers that also waste power before it is transmitted down the OHL.
 

edwin_m

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Am I correct in thinking then that the 3rd rail in the South East is generated as AC, rectified to DC and then inverted back to AC to run the motors?

That sounds like a large waste of money and energy.

Feeding 50Hz from the mains into the type of AC motor used on modern trains will only allow it to operate at one speed, and in practice it probably wouldn't operate at all because it couldn't work at the lower speeds it would need to pass through to get there.

The AC produced for the AC motors has to be varied in both voltage and frequency according to the speed of the train and the power demanded. The power electronics that does this (sometimes known as a VVVF converter) takes DC as input when motoring and produces DC when regeneratively braking.
 

AM9

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On a grander scale, the interconnections across the sea that link the GB mainland power grid to the Ireland and Europe grids are High Voltage Direct Current. For mid-range transmission AC is more efficient, but under sea and at very high voltage there's losses unique to AC and DC becomes the more efficient system. Modern power converters minimise the losses

Between countries there's also the not insignificant problem of ac frequency and phase. A pure ac connector between the UK and France would mean that both countries would need to be in synchronism. That's too restrictive, (both electrically and politically) and multiple interconnectors spaced many miles apart giving unequal length routes would make it impossible.
There is even a DC link between Willesden and Kingsnorth in Kent which allows parts of the South-East to run out of synchronism with each other in an emergency.
 

100andthirty

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Trams don't use 25kV overhead, as this is much more difficult to accommodate safely in streets. Also the low voltage and dc (650V dc to 750V dc) means that trams don't need to carry a transformer around with them. Trams can and do use VVVF inverters to power ac motors.

Having said all that, there are some dual voltage ac/dc trams, generally for tram-train operations. As far as I understand it, the Vossloh tram-trains for the Sheffield trial are dual voltage.
 

thenorthern

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Trams don't use 25kV overhead, as this is much more difficult to accommodate safely in streets. Also the low voltage and dc (650V dc to 750V dc) means that trams don't need to carry a transformer around with them. Trams can and do use VVVF inverters to power ac motors.

Having said all that, there are some dual voltage ac/dc trams, generally for tram-train operations. As far as I understand it, the Vossloh tram-trains for the Sheffield trial are dual voltage.

The new Sheffield trams are dual voltage although for the time being they will only be use for the DC tram system.

For the on board auxiliary power on trains do they use AC or DC? Also how the voltage for the power outlets on bigger trains lowered?
 
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